Project Summary Experience- or use-dependent synapse formation and elimination in neurons are critical for the development and maintenance of neural circuits and information storage. Aberrant control of these processes is thought responsible for numerous neurological and psychiatric diseases. The long-term goal is to better understand the molecular mechanisms by which activity regulates synapse formation and elimination to adjust neural circuit function and behavior. Inhibitory synaptic transmission via GABAergic synapses is critical for shaping network activity and maintaining neural circuit functionality. Defective and aberrant GABAergic synapses are associated with multiple neuropsychological conditions including chronic pain, mood disorders, schizophrenia, and Alzheimer's disease. Like excitatory synapses, inhibitory synapses are highly dynamic and undergo activity- dependent turnover processes not only during early development but also in adult brain. However, molecular mechanisms involved in the regulated elimination of inhibitory synapses are poorly understood. Neuromodulators play important roles in providing flexibility for neural circuit operation and behavior. Although there is a growing body of evidence indicating the important role of neuromodulators in the control of GABA synapses, specific neuromodulator(s) involved in the weakening and elimination of GABAergic synapses remains to be identified. TAFA2 is a brain-specific, novel chemokine-like protein expressed by neurons. Recent studies using TAFA2 gene knockout animals indicate that TAFA2 is involved in anxiety and fear responses. However, molecular mechanisms by which TAFA2 performs its functions in the brain remain unknown. The central hypothesis of this project is that TAFA2 is a novel neuromodulator involved in the activity-dependent elimination of inhibitory synapses. This hypothesis has been formulated based on the preliminary data showing that overexpression and knockdown of TAFA2 had strong effects on the strength and numbers of GABAergic synapses in cultured hippocampal neurons. The objective of the project is thus to characterize and study the function of TAFA2 in the control of GABAergic synaptic transmission and synapse numbers. By using multidisciplinary approaches including biochemistry, advanced imaging techniques, electrophysiology, and CRISPR-Cas9 genome editing, the following three specific aims will be pursued to test the central hypothesis: 1) Examine the rapid modulatory effect of TAFA2 on GABAergic synaptic transmission. 2) Establish TAFA2 function in the elimination of GABAergic synapses in vivo utilizing TAFA2 knockout mice. 3) Investigate mechanisms of TAFA2 action by delineating downstream signal transduction pathway and identifying its receptor candidates. The proposed research is significant, because it is expected to advance and expand the current understanding of the molecular mechanisms involved in the dynamic control of synapse strength and numbers. Moreover, such knowledge ultimately may help to identify new therapeutic targets for neuropsychological disorders.